From this earlier thread it seems your scoot originally had signals, which makes adding some easier.
150 Super rebuild (Page 3)
The wiring diagram for your scoot showed a two-wire flasher, I think without a provision for a flashing I-left-my-signal-lights on light. Two-wire thermal flashers are available, or a three-wire thermal flasher can be used, leaving the indicator light prong unused.
One prong is fed hot, the other prong heads to the turn signal switch. The switch selects left or right, and current heads to front and back lights simultaneously. Lamp current heats the flasher's bimetallic strip and flashing commences.
Your rear turn signal wires should be under the tank in the frame, possibly still connected to the female cowl prongs. Those wires could be disconnected and fed to the rear for some add-on lights. There must be enough wattage to operate the flasher. The cheap flashers will generally flash quickly enough with a 10-20 watt load, up to 50 or more. AC current will also heat them up, but loads become more critical since flashing at idle, while power output is lowest, is important while sitting still, waiting to turn.
Automotive parts houses have the cheapos. Here are some more.
http://stores.ebay.com/Memotronics-Electronic-Components/Auto-Flashers-/_i.html?_fsub=1032868011
I'm not sure which is more dangerous, not having any, or riding while forgetting to turn them off. Wildly gesticulating seems to get people's attention.